Powder materials held together into monoliths with fibrillatable polymers, such as, or most typically, polytetrafluoroethylene (PTFE), but also any other fibrillatable polymers, such as, polypropylene, can be strengthened by the addition of a strengthening cross-connected binder, without the necessity of sintering the primary particles themselves. The result is a ternary system of fibrillatable PTFE, powder material, and strengthened cross-connected binder. A cross-connectable binder is any material which can fuse to itself, including thermoplastic fibers, thermoplastic materials, viscoelastic materials, such as, natural or synthetic rubbers, and any liquid resin or resin combination, such as epoxies and urethanes, that can be cured in situ, either chemically, thermally, by radiation or light, acoustically, or by any other means to form a cross-connected strengthening matrix. Such cross-connecting binders may be heat-fusible thermoplastics as listed in U.S. Pat. No. 5,147,722, columns 7-8, Section C, Binder Materials, and incorporated by reference. Such binders may also include a mixture of silicone elastomers and catalysts as listed in U.S. Pat. No. 4,945,125 and incorporated by reference.
Fibrillatable polymers, particularly fluoropolymers, especially brands of polytetrafluoroethylene (PTFE), known as Teflon, have been extensively described as particle stabilizing agents. The application of shear to mixtures of Teflon and carbon particles form an interconnecting spider web of Teflon fibers which holds the particles together. Such fibers are very narrow, often less than 10 micrometers in diameter. This allows the fibrillated polymer to hold together very small, dust-like particles, which get caught in the spider web-like PTFE filaments. Fibrillatable polymers, such as PTFE, but also polypropylene, can be used to form monoliths or preforms. For example, U.S. Pat. Nos. 4,379,772 and 4,354,958 describe fibrillated PTFE used to bind carbon particles to form sheet electrode material. U.S. Pat. No. 3,993,584 describes general processes for binding together many other different types of particles.
A problem with this method to form sheets or monoliths is that the resultant material is soft in nature and easily torn or broken. PTFE is a weak material, that while excellent at agglomerating particles into an immobilized matrix, provides an easily torn or broken monolithic structure. This makes resultant material more expensive to manufacture, process, handle, and fabricate into other products. For example, winding capacitors, from carbon sheet material, made from fibrillated PTFE and carbon powder requires a minimum tensile strength.
U.S. Pat. Nos. 4,194,040, and 3,864,124 describe the use of fibrillated PTFE to hold together sinterable particles in preforms, which are later sintered together during further processing.
U.S. Pat. No. 5,187,320 describes the use of PTFE to add additional strength to a nitrocellulose binder and high explosive mixture. U.S. Pat. No. 4,337,140, issued Jun. 29, 1982, entitled "STRENGTHENING OF CARBON BLACK-TEFLON-CONTAINING ELECTRODES", describes the use of PTFE to strengthen fibrillated PTFE in carbon particle sheet material.
H. P. Landi, J. D. Voorhies, and W. A. Barber's, "Advances in Chemistry Series", 1969; Robert F. Gould, Editor, page 13, entitled, "A Novel Air Electrode", describes a fiberized, PTFE, carbon powder electrode manufactured with a thermoplastic molding compound that is later extracted.